Vehicle Shudder problems of the 2008 Mazda CX-7

So this is my complaint and story so we went to florida like a week and came back to texas. When we got back home I started to hear an noise on the engine. I took the car to the mechanic and he said that wasn't normal it sounded like a diesel truck. So the next day I had to go to work and I was coming back home the engine light started to flash continuously and car started to shake so I drove it to a auto parts store, so they can check the code and they said it was a piston 3 misfire. I took it to mechanic again and he said the spark plug had oil and he said that was not a good sign he said it might be the engine or a bent valve. First of all this is our only car and don't have money to fix it. A use engine cost 3,000 and up and if its the head it cost like 1900 and I've been reading a lot of bad reviews of this car I really need help I can't afford to fix my car we are a family of 5.